1. Field of the Invention
In the effective expansion and/or drying of tobacco, optimum heat transfer plays an important role in relation to the proper use and effect of heat. To this end, in general increased relative speed between the gas-containing treatment medium and the solid, and a sudden drop in pressure from the higher pressure region to the lower region in various ways have been proposed.
2. Description of the Prior Art
A number of patent specifications describe the use of increased relative speed to improve the drying and/or expansion of moist tobacco. Thus, for example DE-PS 30 37 885 proposes deflection means which are offset opposite in a pneumatic transport tube in order in this way to alter, a number of times, the direction of motion and the relative speed of the tobacco length sections with respect to the gaseous heating and transporting medium. However, these deflection means result in the deposition of tobacco constituents on the walls of the pneumatic tube.
In accordance with DE-OS 36 19 816, it is proposed that the hot tobacco should be separated from the vapor or gas by means of a separator comprising a screen which extends at an angle of 135.degree. to 155.degree. with respect to the axis of the transporting pipe, at the downstream end thereof. An analogous proposal is described in DE-OS 36 19 015, with the difference that the tobacco-drying apparatus comprises two gas flow pipes with two separators which are connected to one another and that porous separators which are each arranged obliquely at an angle to one another should separate the tobacco from the gas. As a result of this system, the tobacco was to be subjected to acceleration twice through the gaseous medium.
The tobacco flowing at an angle and at high speed in the upward direction can be deposited against the face of the gas-permeable separators arranged obliquely with respect to the direction of flow, and can clog them.
With moist tobacco, in particular when the moisture content of the tobacco is above 35%, flow changes in a pneumatic hot-vapor tobacco transporting channel system result in undesirable deposition. This deposition occurs to the greatest extent where the flow change is the greatest, for example in the bent portion of a pneumatic transporting system. This is also the case with the method described in DE-OS 20 14 874. In accordance as a later patent specification of the same method, deposition as a result of cooling or condensation at the bent portion is supposed to be reduced.
DE-PS 38 39 529 describes a process and an apparatus for blowing cut tobacco material, in accordance with which a tobacco carrier gas stream is surrounded by a separately supplied gas stream in order thus to increase a number of times the relative speed between the tobacco material and the gas stream, but because of the previously effected surrounding of the tobacco material by the carrier gas this is achieved only inadequately and not over the entire channel cross-section. The additional gas stream is supplied by way of a plurality of slots which are constructed at an acute angle as an aperture in the channel casing in the direction of flow.
Further treatment of the tobacco after it has emerged from the treatment channel is illustrated only diagrammatically, without describing or claiming the type of treatment or drying. There is no mention of deflecting the direction of flow.
In accordance with DE-PS 33 15 274, a tobacco/gas mixture flows at very high velocity out of a horizontal transporting channel through a narrow nozzle into a dryer tube provided with bent portions. In order that the tobacco can be conveyed further, the flow velocity of the hot gas in the drying tube must be somewhat greater than that of the tobacco/gas mixture emerging from the nozzle. This necessitates an uneconomically large quantity of hot gas and involves intensive mixing or dilution of the tobacco with the gas.
When the tobacco suddenly enters a hot-air environment from the nozzle, the heat transfer caused by the turbulence can bring about an expansion of the tobacco, in particular with tobacco lengths which are more easily expandable. However, the conditions described in the above patent specifications do not sufficiently provide a more significant expansion of tobacco layers in order to bring about the desired expansion effect.
The publication DE-OS 26 37 124 describes the use of a venturi nozzle or a cross-sectional reduction in the tobacco transporting channel in order to increase the relative velocity between the hot gas-containing medium in the tobacco and thus to increase the expansion effect. The expansion effect can of course be further improved by tapering the cross-section of the tobacco transporting channel or by using a venturi nozzle, in the case of tobacco with a relatively high moisture content. The patent specification EP 074 059 is based on virtually the same principle. Here, the additional claim is made that the tobacco material is to be metered and conveyed at the "base point" of a free jet or of a nozzle. This may be ensured, inter alia, by the tobacco metering being directed directly toward the opening of the nozzle, as can be seen from FIGS. 3, 4, 5. The supply of the tobacco or its conveyance in the "base point" of a nozzle cannot be carried out in practice, since the jet flowing out of the nozzle has such a high velocity that it cannot at this point receive the tobacco within it. Only after the jet has widened out and has filled the transporting channel is there a possibility of the tobacco being embedded in this jet and transported further therewith. However, at this point the speed and temperature of the jet are reduced by the widening of its cross-section. This is why optimum exploitation of the heat transfer required for a proper expansion effect for cut leaf tobacco is reduced.
In accordance with DE-PS 31 47 846, equivalent to this patent specification, the tobacco was to be accelerated in the expansion zone and transported at an approximately constant speed, and then decelerated in a divergent flow with an increase in pressure. This construction may be ensured by the temporary narrowing of the channel cross-section, as illustrated in FIG. 1 of this printed specification. Admittedly, the speed acceleration and the reduction of a tobacco/vapor mixture is described in the above-cited DE-OS 26 37 124 by using a venturi nozzle, and "P. B. Dispersionstrockner" cited in DE-PS 22 53 882, that these constructions cannot be regarded as optimum because of the limited possibility of tapering the channel cross-section and because of the mechanical wear of the tobacco by the channel wall.